This invention relates to a light-emitting semiconductor device with a so-called double heterostructure.
A light-emitting semiconductor device is a device intended to create emission of visible rays or near infrared rays by conducting forward current through a semiconductor p-n junction. The demand for it is increasing in various application fields. Among others the light-emitting diode (hereinafter simply referred to as LED) can be used as a point light source such as pilot lamps or light sources forming different shapes of figures and letters such as display panels for use in various equipments such as facsimiles, LED printers, and many other office automation systems. Furthermore, automobile manufacturers are adopting LED as the light source of tail lamps, and many traffic signals are lit by LED. The field of application for LED is increasing in variety and speciality. As one of more specialized applications of LED, there is an infrared radiation unit in which LED performs as the point light source. It is these more specialized fields of application that particularly call for higher output of LED, that is, LED light sources of emitting very intense luminance.
The only compound semiconductors that are put to practical use are those made of semiconductors of III-V groups, and among all of them GaAlAs semiconductor has the highest luminous efficiency and is the champion material for such applications where high illumination is required. The lattice constant of GaAlAs is similar to that of GaAs, and the difference therebetween is 0.14% or so at the largest, so that the deformation caused by lattice dislocation is small and a high quality monocrystal is grown on a GaAs substrate. Further, an epitaxial film of GaAlAs can be grown in both liquid phase and gas phase, and especially the liquid-phase method can produce with less difficulty a high quality crystal of GaAlAs.
For the purpose of obtaining a highly luminous LED from a GaAlAs mixed crystal, a single heterostructure was devised; but it has been known that a double heterostructure makes an LED of still greater luminance. There are several known arts intended to further increase the luminance of the LED products based on double heterostructure. For example, Japanese Provisional Patent Publication (kokai) No. 61-183977 discloses an art according to which a p-type dopant Zn is doped in the active layer at a level slightly higher than the dopant level of the n-clad layer for the purpose of preventing lowering of the luminance, and this is effective enough to make up for the accompanying disadvantage of slight thickening of the active layer.
However, Zn, which is a p-type dopant, has a high diffusion coefficient, such that in a case where the active layer growth and the subsequent n-clad layer epitaxial growth are conducted by a method like the slow cooling method, the diffusion of Zn would outpace the solidification, and cause the p-n junction to shift into the original n-clad layer, whereby the light emission occurs in the clad layer. As a result of this, the wave length of the light emission may become different from the desired length.
Also, since n-type dopants (e.g. Te, Se, S) are liable to separate in GaAs and GaAlAs to thereby cause various defects, it is necessary to keep the carrier concentration low. Further, an infinitesimal but still unwelcome fluctuation in carrier concentration distribution is apt to occur in an epitaxial layer with low carrier concentration grown by liquid phase epitaxy. In other words, a problem exists that the Zn dopant doped richly in the p-clad layer and the active layer diffuses into the n-clad layer, and the said fluctuation of carrier concentration distribution that may occur in the n-clad layer would result in scattered formation of p-type inversion layers, and consequently in formation of thyristor structure.
It has been unsuccessful also to prevent these problems from occurring in the double heterostructure products produced from GaAlAs mixed crystals which are used in semiconductor lasers.
The present invention is intended to remedy the drawbacks of the prior art. It is, therefore, an object of the invention to improve the current and voltage characteristics of an LED to thereby obtain greater emission output from the LED.